Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a core barrel assembly is used to cut a cylindrical core from the subterranean formation and to transport the core to the surface for analysis. Analysis of the core can reveal valuable data concerning subsurface geological formations—including parameters such as permeability, porosity, and fluid saturation—that are useful in the exploration for and production of petroleum, natural gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.
As shown in FIG. 1, a conventional core barrel assembly 2 may include an outer barrel 4 having, at a bottom end, a core bit 6 adapted to cut the cylindrical core and to receive the core in a central opening, or throat 8. The opposing end of the outer barrel is attached to the end of a drill string, which conventionally comprises a plurality of tubular sections that extends to the surface. Located within, and releasably attached to, the outer barrel is an inner barrel assembly having an inner tube configured to receive the core as the core traverses the throat of the core bit and to retain the core for subsequent transportation to the surface.
One conventional approach to preserving the integrity of the core and obtaining reliable formation data, especially reservoir fluid properties such as oil and water saturation, is coring with a fluid retaining functionality, for example, sponge coring. Sponge coring is performed using a “sponge core barrel.” Generally, a sponge core barrel comprises a conventional core barrel assembly, as described above, that has been adapted for use with one or more sponge liners 10. Each sponge liner includes a layer of material selected for its ability to absorb or adsorb the reservoir fluid of interest (for example, oil) from a core sample. Similar to the sponge material approach, there are other ways to construct a material to absorb or adsorb formation fluids of interest. In the context of the present disclosure, the term “sponge” refers to any material that is suitable to absorb or adsorb fluids escaping the formation sample material. As non-limiting examples, this could be material with a porous foam like structure, a felt like structure, a fur like structure, a fabric structure or woven structures, in individual or a multitude of layers, or any combination of the foregoing structures. Also, the terms “absorb” and “adsorb” are used synonymously in this application to describe the capability of keeping formation fluids in a certain location immobilized to a certain degree, even though the technical meanings of these terms are different.
As shown in FIG. 2, a conventional sponge liner comprises an annular sponge layer 12 encased in a tubular sleeve 14. The annular sponge layer 12 is constructed of a material adapted to absorb a specified reservoir fluid of interest. For example, if the particular formation characteristic of interest is oil saturation, the sponge layer 12 may be constructed of an oil-absorptive material such as, by way of non-limiting example, a polyurethane foam. To obtain formation water saturation data, a water-absorptive material is used to construct the sponge layer 12. The tubular sleeve 14 provides structural support for the annular sponge layer 12 and is typically constructed of a relatively rigid material such as, as a non-limiting example, metal. The annular sponge layer 12 is adhered to an interior cylindrical surface 16 of the sleeve 14. Because the sponge layer 12 contacts the core and is relatively flexible as compared to the core, the sponge liners serve to contain the core and protect the core from mechanical damage. Sponge liners are typically supplied in sections, a number of which are placed end-to-end within the inner tube to substantially fill the length (usually a standard 30 feet, although shorter or longer lengths are possible) of the inner tube. The tubular sleeve 14 of a conventional sponge liner typically comprises an aluminum material.
The inner barrel assembly of a sponge core barrel includes an inner tube adapted to receive the plurality of sponge liners 10. During a coring operation, a core shoe disposed at the lower end of the inner tube guides a core 18 being cut into the inner tube and sponge liners 10 disposed therein, where the core is retained for subsequent transportation to the surface and later analysis. A substantially cylindrical interior cavity 20 of the annular sponge layer is of a diameter substantially equal to the diameter of the core being cut, such that an interior cylindrical surface 22 of the annular sponge layer substantially continuously contacts the exterior surface 24 of the core 18 or is in immediate proximity to it, so that any fluid of interest exiting the core 18 will be absorbed by the sponge layer 12 and will not flow off and disperse into the drilling fluid system of the core barrel assembly. The substantially continuous contact between the annular sponge layer 12 and the core 18 often results in the application of significant sliding frictional forces F on the core 18 as the core 18 moves through the core barrel, which frictional forces can, in some instances, overcome the compressive strength of the formation material, causing the core 18 to compact, fracture, jam, or otherwise become damaged. The significant frictional forces between the core 18 and annular sponge layer 12 can also exceed the available weight-on-bit (WOB) applicable to the drill string to which the core barrel assembly is secured, causing the rate-of-penetration (ROP) of the core bit to drop significantly.
When the inner barrel assembly and core 18 are raised to the surface, where the ambient pressure may be significantly less than the downhole pressure, formation gases within the core sample may expand and expel reservoir fluids from the core 18. The expelled reservoir fluids are then absorbed by the annular sponge layer 12 and preserved for later analysis, rather than separating from the core sample and flowing out, as by gravity, from the inner tube. Perforations in the sleeve 14 of the sponge liner allow reservoir gases to escape.